1. Field of the Invention
The present invention relates to a motor and, more particularly, to a motor having hydro-dynamic bearings (dynamic pressure fluid bearings).
2. Description of the Related Art
A spindle motor using, for instance, hydro-dynamic bearings as shown in FIG. 1 has been so far known to experts. In case of a known spindle motor 1 using dynamic pressure fluid bearings as shown in FIG. 1, the lower end of a shaft 4 is fixed to a boss 3 of a stationary base 2 and further, a stator 5 is fixed to the radial outward portion of this boss 3. A small diameter portion 6 is provided on the top of the shaft 4. A thrust plate 7 is fitted to this small diameter portion 6. A bush 8, which is also fitted to the small diameter portion 6, is retained on the top of this thrust plate 7. A cover plate 9 is provided on the upper surface of the thrust plate 7 in the radial outward direction of the bush 8. A sleeve 10 is provided at the radial outward portion of the cover plate 9 and the cover plate 9 and the sleeve 10 are mutually fixed, forming a shaft retainer.
Further, a part of the sleeve 10 retains the thrust plate 7 and is extending downward of the thrust plate 7. A sleeve 10 supports a radial hydro-dynamic bearing 12 which is formed at the middle of the shaft 4. Further, the sleeve 10 supports a rotor magneto 13 at a position opposite to the stator 5 of the radial outward portion of the stator 5. Furthermore, the sleeve 10 has a hub 14 at the radial outward portion. This hub 14 retains a magnetic disk (not shown). Oil is filled between the upper surface of the thrust plate 7 and the lower surface of the cover plate 9 and between the lower surface of the thrust plate 7 and the sleeve 10, forming a thrust hydro-dynamic bearing. Oil is also filled between the shaft 4 and the sleeve 10, forming a radial hydro-dynamic bearing.
In case of such a spindle motor, if the oil is filled in whole area of the thrust and radial hydro-dynamic bearings, the following drawback arises. When bubbles are included in the oil during the oil injection thereof or rotating period of the motor, the bubbles am difficult to be exhausted. Therefore, the bubbles are held in the oil. If the oil includes the bubbles, when atmosphere temperature increases or atmosphere pressure decreases, the bubbles expand, thereby leaking the oil from the thrust and the radial hydro-dynamic bearing, resulting in the shortage of the oil in the hydro-dynamic bearings. To solve the drawback, it is possible to provide a space connected to the atmosphere between the outer surface of the thrust plate 7 and the sleeve 10. However, in this case, oil in the thrust hydro- dynamic bearing flows in the radial outward direction and spreads into the atmosphere by the centrifugal force when the motor rotates, resulting in shortage of the oil. If the oil in the thrust hydro-dynamic bearing becomes short, the thrust plate 7 and the sleeve 10, and/or the cover plate 9 cause a metallic contact and a seizure may be caused in the hydro-dynamic bearing. Further, this type of motor has no reservoir for retaining surplus oil, i.e., a so-called oil reservoir. It is therefore necessary to strictly measure oil quantity present in the bearing portion and if oil quantity is much more than a specified volume, surplus oil may possibly leak to the outside from the bearing portion. Further, as it is not possible to reserve surplus oil in advance, if the hydro-dynamic bearing is used for a long period of time, oil evaporates and in such a case, the oil in the hydro-dynamic bearing portion decreases, and the bearing life may become short.